The invention relates to a filter which includes a stack of foils which are locally attached to one another the rigid member enabling the foils to be moved away from one another in a main direction which extends transversely of the surface, ducts being formed between the foils and also includes comparatively rigid members which are situated to both sides of the stack of foils, extend parallel to the surface of the foils and at least one of which is attached to an outer surface of the stack of foils by way of a buffer member.
The invention also relates to an X-ray examination apparatus provided with such a filter.
In the context of the present patent application a filter is to be understood to mean any system of ducts comprising a number of ducts.
A filter and an X-ray examination apparatus of this kind are described in the not previously published European patent application 98201706.3 (PHN 16.919) in the name of applicant.
In order to form the ducts or channels between the foils the interconnected oils are deformable and are moved apart in a main direction which extends transversely of the surfaces of the foils in the stack, notably of the outer surfaces. The distance between the outer surfaces of the stack of foils then increases; the dimension of the foils in a duct direction which extends parallel to the ducts to be formed remains the same whereas the foils assume undulating patterns in a transverse direction which extends transversely of the duct direction and the main direction and parallel to the surface, the dimension of the stack of foils in said transverse direction being reduced due to said undulating patterns.
The rigid members are only moved away from one another and are not subject to deformation. The buffer member serves to create a smooth transition between the rigid members and the outer surfaces of changing dimensions, so that the foils situated near the outer surfaces of the stack of foils can assume the same undulating pattern as the foils situated near the center of the stack. As a result, all ducts situated between the foils will have the same shape.
In the cited European patent application the buffer member is made of a rubber, a viscoelastic material or an elastic material.
Materials of this kind, however, have the drawback that a tensile force applied thereto in the main direction and a desired shrinkage in the transverse direction are accompanied by shrinkage in the duct direction. Such a deformation in the duct direction, however, is undesirable, because the foil connected to the buffer member will then also be subject to such a deformation, so that the ducts situated opposite the foil will have a shape other than that of the other ducts of the filter.
It is an object of the invention to provide a filter in which such shrinkage in the duct direction is prevented.
This object is achieved in the filter according to the invention in that the buffer member is contractible mainly in a direction extending parallel to the surface and transversely of the ducts.
The buffer member according to the invention incurs no or only negligibly small deformation in the duct direction and is deformed only in the transverse direction. Various embodiments of buffer members having such deformation properties are disclosed in the dependent claims.
The buffer member in an embodiment of the filter according to the invention includes a number of laminations, each of which is rigidly connected, near a first edge, to a plate which constitutes the rigid member, each lamination being connected to one of the outer surfaces of the stack of foils by way of a second edge which is remote from the first edge, the second edges extending parallel to the ducts and being movable towards one another while the foils move away from one another in the main direction.
The second edges of the laminations, for example made of paper or foil, can simply move relative to one another in the transverse direction, without the laminations being subject to deformation in the duct direction.
The buffer member in a further embodiment of the filter according to the invention is provided with a spring comprising turns, the rigid member extending through the turns and the turns being connected to one of the outer surfaces of the stack at a side which is remote from the rigid member.
The turns of the springs can be moved towards and away from one another in the transverse direction. The dimension of the turns does not change, or only hardly so, in the duct direction.
The filter according to the invention is suitable for use in an X-ray examination apparatus which includes an X-ray source and an X-ray detector, the filter being situated between the X-ray source and the X-ray detector. An X-ray absorbing liquid is introduced into the ducts of the filter, the X-ray absorptivity of each duct being adjustable by adjustment of the amount of X-ray absorbing liquid in the individual ducts.